When oil well pipes are embedded from the surface of the earth to an underground oil field, excavation is first performed to provide a well having a predetermined depth and then an oil well pipe, which is called “casing”, is embedded in the well in order to prevent the wall of the well from crumbling. Further excavation is performed from the front end of the casing to produce a deeper well, and then a new pipe for casing is embedded through the previously embedded casing. By repeating such operations, pipes, which are used in an oil field, are finally embedded.
FIG. 1 is a view for explaining the conventional method of embedding oil well pipes. In the conventional method, as shown in FIG. 1, a well having a larger diameter than that of a casing 1a is first excavated from the surface of earth 6 to a depth H1, then the casing 1a is embedded. Then the ground on the front end of the casing 1a is excavated to a depth H2 and another casing 1b is inserted. In this manner, a casing 1c and a casing 1d are embedded in sequence and a pipe called “tubing” 2, through which oil and gas are produced, is finally embedded.
In this case, since the diameter of the pipe, i.e., the tubing 2, through which oil and gas are produced, is predetermined, various kinds of pipes for casings having different diameters are necessary in proportion to the depth of the well. This is because, in inserting a casing coaxially into the previously embedded casing, a certain extent of clearance C between the inner diameter of the previously embedded casing and the outer diameter of the casing to be subsequently inserted is required, since shape failures such as the bending of steel pipes should be considered. Therefore, in order to excavate a deep well for embedding oil well pipes, the excavating area must be increased, resulting in increased cost for excavation.
Recently, in order to reduce the well excavation cost, a method of expanding pipes, after the embedding of oil well pipes in the ground, the inner diameter of the pipes are uniformly enlarged, has been proposed (Toku-Hyo-Hei.7-507610). Further, in International Laid-open Publication WO 098/00626, a method of expanding a pipe made of a malleable strain hardening steel, which does not generate necking or ductile fracture, is inserted into a previously embedded casing and the casing is expanded by use of a mandrel which has a tapered surface consisting of a nonmetallic material has been disclosed.
FIG. 2 is a view for explaining an embedding method comprising a step of pipe expanding. In this method, as shown in FIG. 2, a steel pipe 1 is inserted in an excavated well and the front end of the steel pipe 1 is then excavated to deepen the well in order to insert a steel pipe 3 in the embedded steel pipe 1. Then, a tool 4 inserted in the steel pipe 3 is raised by oil pressure, for example, from a lower portion of the steel pipe 3 to radially expand it. By repeating these operations a steel pipe 2, i.e., the tubing for oil or gas production is finally embedded.
FIG. 3 is a view showing a state where the pipe 2 is embedded by the pipe expanding method. By using the embedding-expanding method, a clearance between steel pipes can be decreased after embedding the pipes, as shown in FIG. 3. Accordingly, the excavating area can be smaller and the excavating costs can be significantly reduced.
However, the above-mentioned embedding-expanding method has the following problems. One of the problems is that the embedded and expanded steel pipe has remarkably lowered collapse resistance to the external pressure in the ground. This means lowering of its collapse strength. Another problem is that the expanded pipe generates bending.
Non-uniformity of the wall thickness exists unavoidably in the steel pipe. The non-uniformity of the wall thickness means non-uniformity of the wall thickness in the cross-section of the pipe. When a steel pipe, having non-uniformity of the wall thickness, is expanded, the thin wall thickness portion are subjected to a larger working ratio than the thick wall thickness portion, so that the non-uniformity of the wall thickness ratio becomes larger. This phenomenon leads to a decrease in collapse strength. Further, the thick wall portion and the thin wall portion of the pipe generate different amounts of expansion in the circumferential direction of the pipe during the expanding process, resulting in different amounts of shrinkage in the longitudinal direction of the pipe. Accordingly, the steel pipe is bent. When a casing or tubing is bent, non-uniform stress is applied to a screwed portion, which is the joint portion between pipes, so that gas may leak.
From the above-mentioned reasons, when the new technology, which is the embedding-expanding method is introduced, a steel pipe having small bending properties, in which collapse strength is not lowered even if the pipe is expanded, is required.